Rate and equilibrium constants for chemical systems involving two- stage processes in aqueous media have been measured and analysed. The major part of this thesis discusses the effect of added organic cosolvents or salts on the reaction kinetics of transition metal complexes in solution. The thesis also includes an analysis of published rate and equilibrium data in order to derive thermodynamic activation and reaction parameters. The oxidation of iodide by the 12-tungstocobaltate(III) anion by an outer-sphere process, of iron (II) by the chloropentammine cobalt (III) cation by an inner-sphere process, and the mercury(II)-catalysed aquation of seme rhodium (III) and chramium(III) chloro-ccmplexes have been studied in binary aqueous mixtures. The aquation of the N,N,N",N" tetraethyldiethylenetriamine-chloro palladium (II) cation and the reaction of cis-bis-(4-cyanopyridine) dichloro platinum (II) with thiourea have been studied in aqueous salt solutions. For the above reactions, it has been possible to dissect the effects of solvent variation or added salt on the rate constants into their initial state and transition state contributions. This has been done by estimating the change in the chanical potential of the initial state from appropriate solubility measurements. The results obtained are discussed in terms of the solvation characteristics of the species involved. The reactions of two anionic iron(II) tris-diimine ccmplexes with hydroxide and cyanide in aqueous media have been studied. Kinetic and spectroscopic evidence for intermediates is reported. With regard to computer-based studies, the tamperature dependence of the rate constant for solvolysis of t-butyl chloride in binary aqueous mixtures has been analysed in terms of the two-stage Albery-Robinson mechanism. The derived enthalpies and heat capacities of activation are examined. The temperature dependence of acid dissociation constants has been analysed using the Gurney equation and the two-stage Eigen mechanism.